This proposal examines the importance and mechanisms of action of S-nitrosothiols (SNOs) in the carotid body regulation of ventilation. We have focused on our recent novel findings that (1) preformed vesicular pools of SNOs in the carotid bodies may be involved in mediating the ventilatory responses to hypoxia, hypercapnia, and acidosis, and (2) systemic injections of SNOs such as S-nitroso-L-cysteine (SNO-L-CYS) elicit Increases in minute ventilation that are markedly reduced in conscious rats with bilateral carotid sinus nerve transection (i.e., rats in which the connections between the carotid bodies and brainstem have been severed) and by putative SNO-L-CYS "recognition site" antagonists. The CONCEPTS driving this proposal are that (1) exocytotic release of vesicular pools of excitatory SNOs such as SNO-L-CYS from primary glomus cells (PGCs) of carotid bodies mediate the ventilatory responses to hypoxia/hypercapnia/acidosis, (2) these vesicles are mobilized by Ca^*-dependent and Ca^*-independent mechanisms involving an array oif fusion proteins, and (3) PGC-derived SNOs exert their effects via activation of specific SNO recognition sites on chemoafferent nerve terminals. Our SPECIFIC AIMS are: AIM 1: To determine (1) the localization of endothelial nitric oxide synthase (eNOS) and the fusion proteins serving vesicular exocytosis in PGCs, (2) whether SNOs exist in cytoplasmic vesicles of PGCs, and (3) subcellular localization of high-affinity binding sites (putative SNO recognition sites) and low-affinity binding sites (putative S-nitrosylation sites) for SNO-L-CYS in the carotid bodies. AIM 2: To characterize the importance of carotid body SNOs in the regulation of ventilatory control and the ventilatory responses to hypoxia/hypercapnia/acidosis, in rats and in transgenic and normal mice. This project will provide insights into (1) the role of carotid body SNOs in ventilatory control of rats and mice, (2) mechanisms regulating the synthesis/storage of SNOs in PGCs in the carotid bodies, (3) the mechanisms regulating the exocytosis of SNOs during hypoxia, hypercapnia and acidosis, and (4) the signaling elements mediating the SNO responses. The knowledge derived from this project will drive future studies directed at testing the hypothesis that free radical-induced damage to SNO release mechanisms and SNO-recognition sites in the carotid bodies participate in the etiology of ventilatory disease states.